I've been using the same strategy. It works also with Griffins who have Furious ability (griffins are better at defence but much weaker at offence, plus demons have 2 very good abilities)
This combo also works great with Target spell that makes all lvl 1-4 units within range attack the Demons/Griffins. Onslaught and Tactics skills from might tree help really much to move griffins/demons closer and faster to the enemy.
I use vampires as support unit when demons are time backed by the end of second turn. Thay have much less work at that time becouse demons usually slaughter like 60-70% of enemy army
